This invention relates to compound-semiconductor integrated-circuit (IC) devices and, more particularly, to a method for making high-speed double-heterostructure bipolar transistor (DHBT) devices in IC form.
DHBT devices are well known and have been proposed for use in a wide variety of important practical applications. Thus, for example, DHBT devices are considered attractive candidates for inclusion in high-speed optical communication and processing systems. In such systems, DHBT devices are well suited to be integrated with optical and/or opto-electronic devices.
It is also well known that the high-speed operation of a DHBT device is limited by its base resistance. And it is further known that a large component of the base resistance originates in the so-called extrinsic base region of the device.
Various proposals have been made to reduce the base resistance of a DHBT device by providing a thicker base layer in the extrinsic region than in the intrinsic region of the device. The proposals reported to date use epitaxial regrowth of the extrinsic base after formation of the base-emitter junction in the intrinsic region of the device. To ensure a good-quality junction, these proposals require either a thick undoped layer to offset base dopant diffusion into the emitter layer (see, for example, "MOCVD-Grown AlGaAs/GaAs HBTs with Epitaxially Embedded p.sup.+ Layers in Extrinsic Base" by K. Taira et al, Electronics Letters, vol. 23, No. 19, pages 989-990, Sep. 10, 1987) or low-temperature regrowth (see, for example, "High-f.sub.max AlGaAs/InGaAs and AlGaAs/GaAs HBTs Fabricated with MOMBE Selective Growth in Extrinsic Base Regions" by H. Shimawaki et al, IEEE Transactions on Electron Devices, vol. 40, No. 11, page 2124, November 1993). Without such special measures, the location of the base-emitter junction and its movement due to dopant diffusion during the regrowth process have a strong and deleterious influence on the current gain of the device.
Accordingly, efforts have continued by workers skilled in the art aimed at attempting to devise effective ways of improving the speed properties of a DHBT device by reducing its lateral base resistance. In particular, these efforts have been focussed on trying to reduce the base resistance in the extrinsic base region in a relatively simple manner and without deleteriously affecting other properties of the device. It was recognized that these efforts, if successful, could increase the speed of operation of high-performance DHBT devices (as characterized by their unity current-gain cutoff frequency and maximum oscillation frequency) and thereby increase the likelihood that such devices would be used as components in a wide array of very-high-speed applications.